The invention relates to the provision of a coating or a thin layer on the surface of a board, in particular, to form an insulative coating on the front and back surfaces of a printed circuit board.
A variety of coatings are applied to the surfaces of many different boards in a number of industrial fields. For example, a printed circuit board carrying printed electrodes to which various electrical components are soldered has its front and back surfaces coated with an insulating, humidity-resistant paint. A conventional coating process comprises the steps of dipping a printed circuit board into a pool of insulating, humidity-resistant paint liquid, pulling the board up from the pool, and allowing an excess amount of liquid paint to be drained by dripping off the board, followed by a drying step. Alternatively, such liquid paint may be sprayed onto the printed circuit board, which is then dried up.
If the coating process in which the printed circuit board is dipped into the liquid paint is used, it is possible that the liquid paint may find its way into the housings of electrical components carrying electrical contacts or mechanically moving parts such as connectors, relays, buzzers or like to provide an insulating film over the contacts or to lock the moving parts against movement. Accordingly, a portion of the circuit board which carries these electrical components is prevented from being immersed into the liquid paint. Specifically, any location on the board which should be prevented from being immersed into the liquid paint is separately coated by brushing or liquid injection with a liquid syringe. It will thus be seen that such technique results in an increased number of processing steps and is also likely to produce an uneven coat layer. Where the liquid paint is sprayed onto the printed circuit board to provide a coating thereon, either the duration of the spraying operation or the flow rate of sprayed liquid must be increased in order to assure that the paint be satisfactorily sprayed into areas located between the limbs of the electrical components. This results in an increase in the paint mist which is diffused into the air or the surrounding environment, requiring a significant installation for the recovery of paints from the air. In addition, the spraying operation causes the solvent used in the liquid paint to be dispersed to result in an increase in the concentration of paints which are being recovered. Accordingly, it will be seen that an increased cost is involved in adjusting the concentration of the paint, in dealing with the solvent dispersed and assuring the environment of a coating stage against contamination.